Hydration and the Summer Heat

Blast from the past issue 212

By Emily Miazga, M.Sc. Clinical Nutrition, 3x Coast to Coast World Multisport Champ and Creator of Em’s Power Cookies


Summer is here and that brings some scorching temperatures with it. Adventuring in hot weather causes a strain on your body’s temperature regulation and physiology, resulting in a decrease in physical performance. There are several ways to reduce the impact of the heat as the mercury rises.


Ever notice that by the end of summer, the heat doesn’t feel so bad compared to the start? That is because you would have acclimatised to it over the course of the season. Acclimatising to the hot weather is actually one of the most important things you can do to counter the effects of the heat on your body. So to speed up the process, make a concerted effort to get out every day in the heat for at least an hour for 1-2 weeks. Make sure that you are well-hydrated when you first head out and avoid becoming dehydrated. What this does is improve your comfort level in the heat as well as your exercise capacity. Another words the heat won’t slow you down as much compared to if you were not acclimatised.  You will start to feel improvements within the first few days, but it really takes a full 2 weeks to get all the benefits of acclimatisation.


This is what happens in your body during heat acclimatisation:

  1. Increased sweating response
  2. Increased skin blood flow response
  3. Blood volume expansion which improves ability to sustain blood pressure and cardiac output
  4. Fluid-electrolyte balance

What is Dehydration?

The most obvious and familiar effect of the heat is increased sweating. The more you sweat, the more water you lose, and this leads to dehydration. When you become dehydrated blood volume drops and the blood gets thicker or hyperosmolar. So the amount of blood that goes through the heart drops and blood pressure regulation is affected. Collectively this places increased strain on the cardiovascular system and the ability to tolerate exercising in the heat becomes more difficult. Dehydration affects performance once 2% or more body mass loss occurs. The idea is that you need to replace these fluid losses by drinking.


There is some debate surrounding the extent as to whether or not dehydration actually affects aerobic performance, and of course there are strong recommendations from sports drink companies to drink a lot. The actual fluid recommendations can vary between just drinking to thirst or using a specific prescribed regimen. I tend to lean towards the more calculated and scientific approach to drinking, especially if you’re going to be out for extended periods of time, and definitely for longer missions. I feel that just drinking to thirst is ‘too little too late’. There are decades of research to support the benefits of staying hydrated and you can’t really argue with the body’s physiological response to the heat.


There is a risk of over-drinking and this is called hyponatraemia, or fluid-overload. This can happen for slower people who are out longer because they are not going as hard so may over-drink compared to faster people who drink less because of the intensity. So the faster people avoid fluid-overload and are more likely to dehydrate, but by that time they are finished and can then rehydrate. Perhaps in response to certain cases of hyponatraemia that have led to deaths, the debate has become polarised and led to the drink to thirst theory.



If you have an upcoming event over summer, or a big adventure planned it is wise to pre-hydrate ahead of time for a few days. Water is fine but you can also use a sports drink with an ample sodium content of at least 500mg/L especially for the 1-2 days before an event because the extra carbohydrate for carb-loading comes in handy.


The recommendation for pre-hydration is 6ml water per kg of body weight every 2-3 hours    


For the average person this will equate to an extra 2.5-3L fluid per day leading up to an event.

Hydration During  

The normal rate of sweating is about 1.0 – 1.5L/hour during harder exercise but some can sweat as much as 2.5L so individual fluid requirements vary. Sweat rate is affected by metabolic rate, the environmental conditions and how well you’re acclimatised. I always recommend using a good sports drink with sodium because that is the main electrolyte lost through sweating. The risk of cramping will be minimised as well by ensuring enough sodium is consumed. For those who are more prone to cramping you can increase the sodium up to 1500mg/L. So for a standard sports drink that contains approximately 500mg/L, you can add another 1/3 to ½ tsp. of salt to the mix. One teaspoon of salt contains 2400mg of sodium.


For during exercise over 1 hour in the heat aim for 250-300mL every 15 minutes, and use a sports drink containing 500-700mg sodium per L. Individual needs vary  


If you are a heavy sweater, you need to consume more, and if prone to cramping add some salt as per the above. Don’t forget about your carbohydrate needs. The target is 30-60g carbohydrate/hour for anything between 1-2.5 hours. If longer than 2.5hours then aim for up to 90g carb/hour. Remember the carbs in your sports drink count towards the carb tally.



After a session in the heat, even if you’ve been drinking during you still need to rehydrate afterwards because this will help your recovery and you need to replace the fluids lost. In order to determine how much you lost, simply weigh yourself before and after and that will be your body mass loss.


The recommendation is to consume 150% of your body mass loss within 1 hour


So if you lost 1 kg, the target would be 1.5L, but this might be hard to get in, so a more realistic target is 100-120% of body mass loss. The best way to recover is to have a mix of fluids and salty foods so you are getting in carbs and protein too.


External and Internal Cooling Methods

In addition to acclimatising and hydrating, you can take things a step further by using cold water immersion, ice-garments or cold wet towels to pre-cool before heading out. I have never heard of anyone actually doing this, although jumping into a cold pool, river or the sea after a hot day is pretty much a universal rite of passage for summer. More practically, having a cold drink or ice-slurry before exercise has a beneficial pre-cooling effect that can help performance, and cold drinks can also be used during to help stay cool.


Towelling off wet skin and fanning is quick, easy and practical if doing an event where you have transition areas. Or your support crew can have a cooler waiting with ice containing an ice-jacket, slurries and cold towels. You will be the envy of all your fellow-adventurers, and the coolest one getting amongst it.


Happy adventuring, Em


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